Upgrade from 2.5 - 3.0 has messed a website up

[wadesigns]

Hi there, after following the upgrade procedure to a T and then login in to the backend of the clients website, I can't do an upgrade to 3.5.1, the site tells me fatal error with isis template.

Can someone advise, help me, may be login and tell me what the issue is please as our client is not very happy.

Big thanks in advance for anyone that can help me

KR

Debs

[Webdongle]

...
Can someone advise, help me, may be login and tell me what the issue is please as our client is not very happy....

You performed the migration on a live site

Use your backup (the one you made when you followed "the upgrade procedure to a T") to restore the original site.

Make a copy of the site on localhost (or your development server) and try the migration there.

The cause of your problem is most likely the result of an incompatible 3rd party extension/template. Third party extensions that are compatible with Joomla 2.5 and 3.5 often use different files depending on which version of Joomla they are installed in. If that is the case then google
migrate joomla 2.5.8 alternative methods

If you didn't create a backup then make a backup now in case attempts to rectify the site breaks it more.

You could try using the files from the Joomla full package to overwrite the files on the server ... but that might not fix it.