site was ok in the morning, then just disappeared later. had NOT made any changes, nothing at all, not even logged in that day.

the admin (backend) pages will show, but only long enough to tell me I'm not logged in.

it's an old 2.0.5 install (yes i know it needs upgrades)

Any ideas? tried the obv. things, repair db from cpanel, reuploaded theme, etc.
DJ.

All thgough it sounds like you tried what you could and even though you said you didn't make any changes I suppose with out any one else making changes on it I don't think it can go from working to broken.. but I would re-track my steps and look to see if you modified any code in the past few days - you said you replaced some files with the org.

But I know from experience and I'm not pro at this stuff but even if you change the "Language files" with something that it does not like you will get blank pages.. Check all coding even posts its in the files them selves and not in your DB i believe so check your templates and .php files. Hope this helps..

Beyond repairing the db did you also try to empty (not drop) the sessions table?

Did you switch on(faq) php debugging to look for additional error messages?