Does it simply not take a charge? Or does it die more quickly these days? If it dies too quickly, check to see if it's NiCd (not NiMh). NiCads can potentially develop 'charge memory' under heavy usage. (Virtually non-existent with AA or AAA batteries, but heavy duty ones sometimes build up.)
NiMh batteries are not subject to charge memory. So if yours is a nicad, then leave it in your drill, lock the switch on, so that the battery will discharge as much as possible (an hour or two). Then give it a full charge.
If it's simply not taking or holding a charge, then it's time for replacement. Rechargeables don't last forever, and even low usage will still die out over a few years. You can spend money on info to rejuvenate them, but it's a waste. They'll give you some pseudo-technical crap that's theoretically possible; hey, it's theoretically possible for me to swim to Japan. But in reality, it won't happen.