Perhaps with a little more information/clarification.
Php version?
When you say you uploaded the ZC 1.5.4 files, did they go in a "new" directory or did they simply "overwrite" the existing?
Until the db upgrade is complete, accessing the admin won't work using ZC 1.5.x files on a 1.3.x database... If you had multiple profiles on your 1.3.x version, you may need to create a new temporary user through the guidance of the following, applying the 1.3.9 version of the fix:
http://www.zen-cart.com/content.php?44
There are potentially other things to do, but more info is needed, in particular the remaining answers to the posting tips questions.
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