just downloaded transactions from bank (Royal Bank of Canada) andthey posted automatically for some reason, but the real issue is that there were hundreds of them so account file is totally messed up. Is there anyway to undo that or do I have to start from scratch?

You have a couple of choices:

• In case you have set the number of backups to keep on disk different from zero you might use one of those files to start with. In case this is set to zero you don't have such a file.
• The imported transactions usually are marked with a yellow background. If they still are, you can use the status filter box in the to right hand corner of the ledger view and select the imported transactions. Then use Ctrl+A and delete to get rid of them.
• In case you already accepted them, you can manually delete them.

Unfortunately, we do not have an 'undo' feature.
Hope that helps.