oh hang on.....is 120 film use so rare now that we forget what it looks like or how this format is ?ha hah a
regular 35mm film is contained in a sealed film container and accidentally opening camera will affect the exposed portion of the film leaving the balance inside canister unexposed (normally 2 frames fwd is sufficient to continue shooting) and the exposed images very likely suffering from light leak streaks
120 film however has always come with light-leak proof paper backing along the whole roll and the open end is loaded onto a take up spool.....
So when camera back accidentally opened normally the last 2 exposed images will be affected .... the rest of the roll should be ok to use.... just fwd 2 frames (u are using 645 so can afford it... if you're shooting in 690 then take Ortega's advise and just keep old roll and reload a fresh roll)... and the already previously exposed images should be safe and sound except for the last 1-2 exposed images that have not been rolled into the take-up spool yet
less than 10 serious years into digital and we have forgotten the old ways LOL !
remember your 50 year old 6x6 or 690 cameras with little red tinted windows on the back itself to count the number printed on the backing to tell u which exposure you are at ????