More info might be useful here to pinpoint the possible source of the problem:
- What model / region of PS3 you have?
- What version of FW it had before?
- Have you used any CFW / JB before?
- Have you upgraded hard drive in it?
- Any faults or problems before - either with hard drive ot BD drive?
As a start - I'd try to go recovery menu and do the database rebuild or reinstall the the same FW you have at the moment (the one you're trying to update from) in there and then try the update again or you can also try to run 3.61 update directly from recovery menu. If you can still get back to XMB with the current / old CFW it might be worth to try to reformat the hard drive (but indeed losing all it's content so do so only if you have backed up everything or if you don't mind).