1991 Mazda MPV Question: my vehicle don't want to start, but the engine still cranks?

For the past 2 months my vehicle struggles to start. I have to keep cranking the engine for about 3 to 6 seconds before it starts, and when it starts the temp. gauge has to be in the middle in order for me to turn off the car. If the temp. gauge is less than the half point, the vehicle will not start again for 2-3 days. About 3 days ago, my vehicle started fine. I drove it around the block to warm the engine up then went home. After an hour, I wanted to go to the store and when I went to start my vehicle. My vehicle wouldn't start anymore. The engine cranks but it won't turn on. What could be the Problem?