2000 blazer wont go into 4x4

When shifting into certain gears certain things are required, ie for 4lo u must be travelling under 5km per hour and in neutral. this varies with manufacturer but is used to prevent damage. if u know this then forget what i just said. There is a motor on the side of the transfer case which changes the range in which your truck is in, and if not used for a while, it will seize up. it soulds like this has happened you yours if it is indeed the one im thinking of. easy to test. have someone push the button while u listen. make sure u block your tires, and do a quick check for power at the connector with a test light. hope this helps

your truck works on a vacuum system check on the fire wall toward to center driver side of the truck. you will see a little black box with vacuum lines that box switches the transfer case.....

oh it is under the hood sorry for not listing that before also just make sure vacuum lines have not cracked or pulled off..

If it is the S10 blazer then the problem is probably your TCM. I have the same issue from time to time. Easiest fix is to disconeect battery for 10 mins. Then reconnect. This seems to reset the computer and all is good for a while.