I do not practice in IL, but I may offer some general advice. First, no one gave you a guilty plea. You pleaded guilty of your own free will. Courts generally establish this when you plead before you are sentenced.
If you do not complete your community service, the deal you negotiated for probation may be lost. For example, if the case would be dismissed upon successful completion of probation, dismissal would no longer occur.
Additionally, your probation could be revoked. If your probation is revoked, you could be sentenced up to the maximum allowable penalties for the crime(s) that placed you on probation in the first place.
IMO, it is better to do the community service. If there is a reason you cannot complete the community service in the time allotted, contact your probation officer and explain the situation immediately. See if you can work something out. If you are on unsupervised probation, contact the court that sentenced you.
First of all, "they" can't give you a guilty plea. Only you can enter a guilty plea.
Before anything can happen to you the prosecutor will have to file a petition to revoke your supervision. If you don't complete the community service, OR the prosecutor can prove that your failure to pay the fines/costs is "willful" (that you have the ability to make some kind of payment and fail to make any good faith effort to pay what you owe), then you can have your supervision revoked and a judgment of conviction entered against you. The best case scenario you get an extension of your supervision and time to complete the tasks you were given to do. The worst case depends on what you are on supervision for (unless it is classified as a petty or business offense you could go to jail), and whether they are state charges or municipal (city/village) charges (as some municipalities' ordinances do not carry the possibility of jail--and shouldn't result in jail if you didn't have legal representation at your guilty plea hearing unless you were advised of the possibility of jail and were given the right to ask for counsel if you could not afford an attorney).
If the prosecutor files a petition to revoke, you will receive notice by mail. If you have moved since the day you pleaded guilty you need to make sure the Clerk and the prosecutor have your correct address or a warrant could issue for your arrest. It won't be a defense that you didn't get notice of a new court date because you moved. There is a presumption that you are living at the address they last had on file for you. The burden is on you to keep it updated. If you receive such notice I would strongly suggest going into court with an attorney.
If you fail to complete your agreement and are violated, the judge can sentence you to any sentence that was available prior to you pleading guilty. That may include incarceration.