I spent quite a while tagging my files then I left them on mp3gain overnight and now I check and there's no tags at all. Opening them via VLC shows that everything is still there such as Title, Artist, Track Number, Cover Art and Album Artist. Is there any way to restore it so that Mp3tag can read the tags again???? 
MP3gain likes to write some fields as APE tags so that APE tags and ID3V2 tags get out of sync.
As MP3tag internally sets
APE>ID3V2>ID3V1
you see only the contents of the (asynchronous) APE tags.
So if you switch off the reading of APE tags, then the information should be there again.
In general you should try to avoid APE tags. and preferably get the contents into ID3V2 tags.
I set my mp3gain to ignore tags to avoid this happening again but is there anything I can do for the files that already lost their tags?
I am not too confident.
If you have set MP3tag to write ID3V2 tags and there was no content in the corresponding APE fields then this is what you will get: no corresponding contents.
If you have only read the files, then set MP3tag to ignore APE tags while reading (which will then loose the information in the APE tags) and see what is left in the ID3V2 tags.
Empty is empty.
Actualy, I do not know why MP3gain should ignore tags - it should not write APE tags that would be it.