It could mean one of two things, but out of context it is unclear. This can mean "I forgot (something)", but if you mean that, you would normally mention the thing you had forgotten. It can also mean "My mind went blank," but normally you would say "spaced out" instead of "spaced" for this meaning. That is, if you say "I totally spaced out," you probably mean "My mind went blank." If you say "I totally spaced (something)," you probably mean "I forgot (something)." For example: "Did you remember to study for your test?" "No, I totally spaced it." This means I forgot it (I forgot to study for the test). or "Did you listen well today in class?" "No, I totally spaced out." This means my mind went blank (and therefore, I did not listen to the teacher.) You could just say "I totally spaced," for either of these, and it would be clear what you mean in the context.

It means "my mind went blank".