What actions should I take, I haven't filed a tax return in 20 years

Your problem may not be as big as you imagine. The thing to do likely is to sit down with a tax professional (Enrolled Agent, CPA, or attorney) and see what taxes you really owe.

If you are an employee and fill out the W-4 for your correct number of exemption, your employers may have withheld more than the taxes that you owe, in which case, the IRS will not care that you have not been filing.

Have you received any notices from the IRS letting you know that the IRS thinks you owe taxes?

The first step is get an assessment of your situation with a professional.

Mr. Nguyen is right on target. There was a recent report on NPR about the IRS and its willingness (because of the economic issues surrounding and affecting all of us AND the gov't) to deal with people (i.e., possibly agree to a compromise on your taxes, if past due). It's always best to get a tax attorney to help you with this. First, of course, you need to do the taxes for each missing year and determine where you owe and where the gov't owes you. (Needless to say, if you owe and you're late, there are penalties and interest; if they owe and you hadn't claimed, you won't get either penalties or interest!) Once you determine the bottom line, a seasoned attorney can approach the IRS with an offer of compromise on your behalf.

The good news is that you may not be required to file all of the missing tax returns to be considered in "compliance". Often only 6-7 of the most recent years of returns need to be done as well as any previous that the IRS has incorrectly completed that you want to challenge.

Talk to an attorney that focuses his or her practice on IRS resolution issues.

If you have lost the W-2s sent to you, you can contact the employers to see if they would re-send the W-2s to you. Otherwise, the IRS has the information that was reported by the employers. The IRS would provide you with the income information provided to the IRS.

You need professional help. This is not a do-it-yourself project. Interview several tax professionals to see if they have handled matters of this magnitude in the past. If possible, get a reference or two that you can contact. I recommend using an attorney, licensed in tax court, because that type of representation may become necessary.
The statute of limitations on filing a return NEVER starts to run until you file the return.