Craig’s response is spot-on.
Have a go yourself. You should have access to some form on CMS to do this.
Secondly, check the agreement you have with the supplier. The rights should be with you and they should support a migration. Some companies might charge for larger migration work but all professional service suppliers should enable handover or hand back.
Lots depend on what type of solution you are using and the supplier.
If you get into a tight spot, drop me a line at email@example.com. Almost 50% of our work is recovering or rebuilding work done by other parties that have not met expectations.