WHen I try to boot my ThinkPad I get the error "UNMOUNTABLE_BOOT_VOLUME". I have not had problems in past nor have I made any recent changes. Any suggestions to get back on line. My OS is windows XP PRofessional

That error normally implies that your hard drive has terminated itself and it is completely unrecoverable.
Sorry about that but normally your only course of action is a complete reinstall of your OS, if your really lucky a repair may work.

Have a look at this also which relates to your problem

http://support.microsoft.com/?kbid=297185