Where is the crowbar in black mesa?
Solution 1:
Just keep going. The crowbar is a little bit later in Black Mesa than it was in the original Half-Life, but it's impossible to miss it and it's not that far away. It's pretty linear at the beginning, so just go whichever way is available until you get it.
Solution 2:
Head back to the main entrance ( the terminal where the guard complains of the system crashing) and find the ventilator shaft blown open. Crowbar is wedged in the door handles.
Just run past all the zombies you first encounter - you won't have enough bullets for all of them