Just registered and thought I should add my comments as I just had this problem.
My 2000 A4 would pop to the safety latch but the bonnet release catch was seized.
To get the bonnet open I gently removed the chrome trim around the front grille. It has clips on the inner side so gently prize the outer egde with your fingers till it all comes away.
Then remove the front grille by pulling the top out. You may need to use a flat head screwdriver to help with this so use some card so as to not damage the bonnet. It will eventually pop out and the catches that hold it in shouldn't be damaged. If you see where the catches are you can push them down as you pull the grille forward, that might help.
The bottom of the grille does not pop out, you lift the grill up and out once the top comes free.
Then you can easily see up to the catch, spay it with WD40 or release agent and it will eventually loosen up.
Then just grease up the mechanism so it won't happen again.