You need to become familiar with general middlegame and endgame principles so you can assess the strengths and weaknesses in your position and your opponents. If you don't know general rules of thumb like a Rook on the 7th rank is worth a pawn, trade pawns when a pawn down and pieces when a pawn up, and the easiest endings to draw are those with opposite-color Bishops you need to learn them - and lots more!
First read My System by Nimzovitch, next read something by Euwe, Fine, Kotov or Romanovsky - they all wrote excellent works on middlegames and general strategic principles.
You should also work on endgame principles, try reading a general endgame book by Chernev, Euwe or Keres for starters. Fines Basic Chess Endings is indispensable as a general endgame reference book, but it's not meant to be read all the way through anymore than a dictionary is.
How do you cope with going blank, so to speak, and not know what to do next during a game? Or am I the only one that faces this problem from time to time?