URGENT: Lost Profile or similar

I am running a WIN XP Home edition.

I work with some databases and had to download a document, but I was not able to as I was told I do not have sufficient privileges and to speak to the administrator.

I am the administrator…

I was able to download from this database last night, but not today. As far as I am aware there were no changes.

I rebooted and suddenly I can only log in as guest and cannot log as the main account, as it is not there.

How do I put it back?
Why would it be deleted?

Any replies are welcome, thank you.