I upgraded my quickbooks account to the self-employed version and now I can't access the information in my old account. How can I access the old account?

There is no lesser product that QBSE so you could not have "upgraded" from any of the  four QBO versions to QBSE but that being said, your previous company file must be exactly where you left it with all its data in place. Your master Intuit log in should offer you access to each of your accounts.

If you were able to so-called upgrade from a double-entry system like QBO into the single-entry systmem of QBSE there is no way the data could migrate