Steps are:
Step 1: Save your blender project
Step 2: File --> New --> General
Step 3: File --> Open (find your messed up blender project)
Step 4: Go to the cog (settings) in the top right
Step 5: Uncheck 'Load UI'
Step 6: Load your blender project
It will then have default UI
I only remembered this was possible, not how to do it. The steps above are copied from this persons lovely succinct video tutorial on how to do it.
