It can happen to any chess player...

It can happen to any player without warning!  All of a sudden you look at the game as if you have never played it before, you are unable to string together any form of cohesive check-mating attack, and even the most basic strategies and tactics seem difficult to comprehend. 
Your defenses go to pieces; you do not castle early enough or you make other elementary blunders.  You start losing major pieces because you do not realize that they are under attack until it is too late.  Moreover, this becomes a trend and not just the occasional one-off, and after too many games you just wonder what is going on.
These are strong indications that you have contracted the chess player's equivallent of writer's block!!  Chances are that your chess playing brain has come to a major cross-roads, where it is telling you that you may have reached the highest level that you ever will do; that is, the fairly good level of play that you had reached prior to this affliction.  I repeat, may have... 
But what are the reasons, and can you ever get out of it?  The reasons are probably that you have found standard openings, etc, to be ineffective or inconclusive against any decent player, and that even if you yourself do not make any major blunders, the best situations that you end up in are more often than not, tit-for-tat type games where any real progress becomes very difficult; because both yourself and your opponant/s have been sticking to standard booklines - and so your chess playing brain, after a while, may simply give up.  This usually will happen to good average club-level players; who didn't learn the game early enough or consistantly enough to open the right mental pathways to become very strong players, but there can be another, very common reason, and luckily a way out of this syndrome.
You may be playing too much chess, and have gone into overload!  You have become chess-brain dead!  Remember, when you play chess, you are not just playing chess.  The way out maybe to reduce the amount you are playing for a short period or so; that is the number of games you play in a day, and or the number of days you play in a week, and so forth.  This should recharge the batteries and get your cogwheel's back into gear.