FIRSTLY - IS YOUR COMPRESSOR RUNNING? 

IF YES - THEN MOST LIKELY YOU HAVE EITHER A DEFROST ISSUE - TO CHECK IF YOUR EVAPORATOR IS FROZEN SOLID ALL THE WAY DOWN THEN YOU MOST LIKELY HAVE A DEFROST ISSUE 
THINGS TO CHECK
1. DEFROST TIMER OR CONTROL BOARD (DEPENDING ON YOUR MODEL) YOUR DEFROST ELEMENT AND YOUR DEFROST TERMINATOR / THERMOSTAT
DEFROST ELEMENT - CHECK FOR BREAKS, OHMS AND/OR CONTINUITY WITH A MULTIMETER. IF YOU HAVE ANY READING BETWEEN THE TERMINALS THIS GENERALLY MEANS IT IS OKAY. 
2. DEFROST THERMOSTAT / TERMINATOR - *IF COLD AND COVERED IN ICE - SHOULD READ CONTINUITY BETWEEN THE 2 WIRES. IF YOU HAVE IT OUT OF THE REFRIGERATOR ALREADY HOWEVER LOOK FOR WHERE THE WIRES GO INTO THE THERMOSTAT, IF IT LOOKS SLIGHTLY POPPED UP OR OUT, THERE IS A GOOD CHANCE THIS IS WHATS BAD. WITH THAT SAID, THESE ARE OFTEN VERY CHEAP TO REPLACE SO "IF IN DOUBT - CHANGE IT OUT"
3. DEFROST TIMER - MOSTLY WHEN THIS GOES BAD THE TIMER STICKS IN DEFROST CAUSING THE THE COMPRESSOR NOT TO START SO BY TURNING IT MANUALLY OUT IF THE COMPRESSOR STARTS THEN THE TIMER IS MOST LIKELY BAD. A DEFROST BOARD HOWEVER IS A LOT HARDER TO DETERMINE UNLESS YOU SEE A PHYSICAL BURN ON THE BOARD SO BASICALLY IF YOUR DEFROST ELEMENT AND THERMOSTAT TEST OUT OK THEN THE LOGICAL PROBLEM WILL MOST LIKELY BE THIS. 

IF COMPRESSOR IS RUNNING AND YOUR EVAPORATOR HAS FROST OR ICE IN ONLY ONE PART OF IT OR NO ICE AT ALL THEN YOU HAVE LOW OR HAVE LOST YOUR REFRIGERANT CHARGE. *THIS MEANS YOU HAVE A LEAK IN THE SYSTEM. 
*BE CAREFUL ABOUT PAYING SOMEONE TO JUST "TOP IT OFF AGAIN" AS IN SOME CASES IT IS ILLEGAL TO DO SO (DEPENDING ON THE REFRIGERANT TYPE) AND OTHER CASES - THE REPLACEMENT CHARGE "UNLESS THE LEAK IS REPAIRED" WILL JUST LEAK OUT AGAIN. DEPENDING ON THE SIZE OF THE LEAK CAN MEAN ANYTHING FROM A DAY TO A MONTH TO A YEAR. 
*THERE ARE SOME "LEAK SEAL AGENTS" AVAILABLE ON THE MARKET HOWEVER IN MY OPINION THEY GENERALLY DON'T WORK WELL OR FOR VERY LONG AND ARE NOT PERMANENT SOLUTIONS BUT CAN GET YOU OUT OF A BIND UNTIL YOU HAVE TIME OR CAN AFFORD TO FIX OR REPLACE THE REFRIGERATOR OR AIR CONDITIONER . I HAVE FOUND THAT "THE GUARANTEED TO FIX YOUR LEAK" ONLY WORKS AROUND 20% OF THE TIME BUT CAN SLOW THE AMOUNT OF TIME IT TAKES TO LEAK BACK OUT. THERE ARE ALSO SOME LEAK STOP AGENTS THAT SAY THEY ARE SAFE TO USE BUT I HAVE SEEN THEM DAMAGE SYSTEMS AND/OR CAUSE IRREPARABLE DAMAGE TO COMPRESSORS OR OTHER COMPONENTS IN YOUR SYSTEM SO MAKE SURE YOU DO YOUR RESEARCH BEFORE USING ANY OF THESE AND JUST BE AWARE THAT THIS MAY JUST BE A QUICK CHEAP TEMPORARY FIX WHICH MAY OR MAY NOT WORK FOR LONG OR AT ALL.