Turned it on and three minutes later it turned off. Not working at all

The first thing I suggest is to check if  you have a 'Ground Fault Interrupter' (GFI) protecting that circuit; they will sometimes trip because they have misinterpreted a current condition that some appliances may do randomly. These look a little different than the standard outlet and have two buttons, one test and the other reset. Some have small indicators that light if the circuit has been disabled.    There is normally only one in a kitchen or bathroom.   If you don't find this, plug in another appliance briefly (a nightlight will do) to se if there is voltage at the outlet used for the micowave.
If the outlet is dead, it's time to locate the main breaker or fuse box and check to see if there are any tripped or blown.
If you don't find any external problem then it is time to pull the AC plug and remove the back panel. This normally won't require more than the removal a 5  or 6 screws.
Check the area around where the AC line comes into the cabinet for a fuseholder which are often easy to find.  Typically, these appliances use a standard US style fuse except that the body may be white (ceramic) instead of glass. Keep fingers away from everything except the fuse as there are parts that can retain a nasty charge for hours.
If you have a multimeter, you can check the fuse for continuity; if you don't, then you will need to find a replacement fuse of the same rating which will be in the range of 10 to 20 amperes and this is stamped into the metal end caps. Don't replace any ceramic-body fuse with glass since a fuse can produce a high gas pressure when it blows and glass will often shatter.
Any appliance service center will have this type of fuse in stock and it shouldn't cost more than a buck even when business is slow. Buy two, not just a single one.
If you replace the fuse and if fails again, you will have to let a tech do some magic and locate the cause of the overcurrent that is blowing the fuse.
There are a number of parts that can fail that don't cost a fortune but if you bring it in, or have it serviced at home, ask for the type of part that has failed if the cost exceeds $10 and post back here so I or another tech can tell you if it is a fair price.
Good luck-