You got yours working but perhaps what happened to me will be helpful to someone else with a blank admin.php.

My admin.php suddenly was blank after working fine for weeks. Well header and top links were visible but I had no admin menus. I used phpMyAdmin to turn debug_mode on in the xoops_config table. The resulting error messages then led me to discover that my cache directory was for some reason no longer writable. Apparently the system writes adminmenu.php in there. Fixing permissions made everything work again. What I don’t know is how or why the perms on the cache directory got out of whack in the first place, since I had made no changes to the site.