With the limited information provided, it appears that the timer is faulty and should be looked at.
When you move the knob to begin a wash, try to get your ear next to the timer knob and listen, can you hear the slight noise of the timer motor running? If not, try moving it to another cycle such as Delicate, etc. and listen again. If you can hear the timer motor running in any other cycle, it could be that the motor is fine, but the plastic cycle gears, used to set the wash/rinse cycle times, has worn out.
You can remove the top panel to get at the timer by removing the screws on the front and back of the top mount panel. Once you've done that, you can take a better look at the timer motor and again listen to see if the motor is running during the wash cycle that isn't starting for you.
If after all this, it is likely that the timer gears for your main wash cycle have either worn down or been damaged. You will need to replace the entire timer mechanism, as the plastic gears can't be replaced separately as the many small parts are a nightmare once they come apart, (sorta like a Rubic's cube but no colors to help you figure out where they go!)
The part number for the timer assembly is #8577356 and costs between $48-$62 depending on where you get it.
Here's a line diagram/drawing that shows the washer so you can get a better idea of what's involved to undertake this relatively simple repair.
The drawing might differ slightly, as there are two (2) series of this washer, you need to check your model number to see if the last letter (part of the preprinted part of the label/tag) is either a "O" or a "T". The following drawing is for the "O" series, just change the number to see the other one, if you need to.
Just be sure to unplug the washer from the outlet before attempting to remove and replace the timer. It's straight forward with simple wiring plugs that you simply unplug off the old timer and push onto the new one. The only other thing you'll need is a socket to remove the locking nut and toothed washer that holds the timer to the front control panel, just pull off the cycle knob and then the nut & washer to free it from the front panel.
By the way, the timer is the same part on either series of this washer, so regardless of what version you have, the timer part # I provided, with work with both washers.
Hope you find this Very Helpful and best regards!