It is common to find fawns left alone for long periods of time by the doe (the female parent). The doe knows when to return and will not do so if humans are around. The best thing you can do if you find a fawn is leave it alone for 24 hours or put it back where it was found, unless it is truly injured or orphaned. Even if it is injured or orphaned, it is okay to put it back or leave it alone.
If you are truly concerned that the fawn is injured or orphaned, but not sure, read the following: