Hi my internet company had changed my serivce without telling me because they decided to no logner offer that plan and sent me a huge bill based on the new service. They said they sent a letter but I didn't get anything. Can they legally do this to me?

Most likely no, I would guess the notice was one of those "legal" notices that come in small print in your bill that known of us read.

Also since normally the bill is for the up-comming month, you merely cancell the service unless you had a contract for a specific time,
But go find a new company and change.