Fatal error: Allowed memory size

I have just installed a fresh copy of WordPress and then when I tried to upload a new theme the site logged me out and now when I go to login at wp-admin I get the following error message:

Fatal error: Allowed memory size of 262144 bytes exhausted (tried to allocate 274012 bytes) in /home/cjsukimp/public_html/wp-admin/index.php on line 13

I have been into File Manager and the theme has not been added to the files and nothing else has been changed on the site?

Anybody have any ideas?