Virtually all government employers (every jurisdiction; every "level" of government -- state, local, federal) have standards and procedures in place regulating the potential return or recall of retired employees. These standards and procedures are usually set forth in statutes, administrative regulations, or administrative rules promulgated by a statutorily-authorized rule-making authority. The specific provisions of your former employer's legally-authorized standards and practices will govern your situation because employing entities do not ordinarily have any discretion to act except as consistent with the statutory or regulatory provisions. So, the first step is to locate and scrutinize that body of controlling regulation.
Because it can be a highly technical and complicated task to identify the universe of applicable statues and regulations, and because interpreting those laws and regs can be very challenging, I strongly recommend that you rely on skilled and effective local counsel for these tasks, hopefully an attorney with extensive experience in the law of retirements involving your specific former employer.
In most circumstances there is a relatively short period of time in which a retirement can be rescinded and unravelled. 90 days is very common, and I have seen some regulatory schemes that allow rescission during the first three years. These time limits will most likely be mandatory and inflexible, and they may vary depending on the nature of the retirement (service, disability, service-related disability, etc.).
Where return to work is barred by the character of the retirement, it is sometimes possible to effect a change in the type of retirement and that can make the retired worker eligible for return to work or for new employment.
Where the time for unwinding the retirement has elapsed, sometimes the employer has the ability to hire a retired worker as a new hire. This used to be a lot more easily done; scandals about "double-dipping" in the last many years have made the ability to do it much less common and have also made government employers far less willing to make this kind of effort. Before you invest time or resources in making applications for new employment, or taking any qualifying exams, check your entity's civil service and personnel rules to make sure that you are eligible for new employment and take careful measure of any impact or consequences to your retirement. Retirement plans often have very technical, unfair, and hard to decipher rules that are nevertheless legal and enforceable and you do not want to step into any traps that could affect your present retirement or your credit toward a different character of retirement.
In all candor, these statutory schemes are very very impenetrable and you need counsel -- specialized legal counsel. Legal counsel can assist you in understanding whether you are eligible, and perhaps in making you eligible for rehire or new hire if you are not presently eligible. But then there will be the looming issue of whether the employer wants to rehire or new hire you. That often depends on the nature of the work and how long the worker has been retired. If the technology and standard practices of your field have evolved past your skill set, you will find that all the eligibility in the world does not get you rehired. Ditto if all the folks that you worked with and knew have moved on, or if your employer was pleased to see you go. The employer is not obligated to rehire you and principles of discrimination law are very unlikely to affect that fact.
Please be very careful and accurate in doing a sound cost-benefit analysis/projection before you get too deeply invested -- financially, emotionally -- in this matter.
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If you were represented by a union then contact them. If you signed an "early retirement" agreement of some type then there might be a contingency clause within the document that addresses unforeseen events, etc.... It does not look like your doctors committed malpractice from the facts you stated. It does not look like the medication you took failed to notify you of the possible side effects given the facts you relayed? Contact all the employees with whom you had a good working relationship, use them as references, and have them recommend you as a top candidate for any new position to which you apply? Take as many civil service exams as you qualify for and score high? Good luck.
Not legal advice / No lawyer/client relationship.