PSQL command prompt disappears after entering ALTER TABLE statement with semicolon

I'm trying to add a unique constraint to an existing table using the PSQL command line:

ALTER TABLE table_name ADD CONSTRAINT test_key UNIQUE (col1, col2);

The problem is that after I hit enter, nothing happens. I just get a new blank line without the command prompt, but the cursor is still there. I have to eventually hit ctrl+C to cancel the statement. Can anyone tell me what I'm missing?

It turns out it was running the script. There was nothing wrong with my script, it just didn't give me any indication it was running, other than me being able to enter in to new blank lines and the cursor blinking. It took a while but it eventually created the constraint.