Getting references after a company closes after 4 years as a dental hygienist

The company closed and I am not sure how to reach former colleagues. I have read a lot of threads here and could not find my exact situation. A colleague offered to vouch for me. How do you ask politely?

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.

Ask former colleagues and supervisors in a short message and give them a summary of the job you are applying for.