In some common fixed expressions to do with place, time and movement, normally countable nouns are treated as uncountables, without articles. Examples are:
To/at/from university/college (GB)
To/in/from college (US)
To/in/into/out of bed/prison
To/in/into/out of hospital (GB)
With place nouns, similar expressions with articles may have different meanings.
- I met her at college. (When we were students)
- I met her at the college. (The college is just a meeting place.)
In American English, university and hospital are not used without articles.