The better news is that it might be as simple as an exhausted CMOS backup battery. Go to the Lenovo website (www dot lenovo dot com) and search on your full model number (2635-3au, for example) and hunt around for information on replacing the CMOS backup battery. If you have some luck and ingenuity, you may be able to replace the existing backup battery with its watch battery equivalent and get back up in a relative jiffy.
The less good news is that the hard disk may have decided to give up the ghost. If the computer can boot from any MS-DOS floppy disk and the computer can remember what day and time it is, then the system board and CMOS battery are probably okay.
Then a dead hard drive becomes the more likely case
It then becomes a matter of how much ($ & hours) you want to spend to restore a venerable old computer... maybe $50 or so for a hard disk, plus hunting on ebay for a Win98 operating system to reinstall, and then your time rebuilding...