Your first sentence may be your answer.

Leaving the gas in when not in use gums the whole works up, you should have fired it up at least once a month and run it for a few minutes. Been there done that :(.

Do you have your owners manual still? If left for any length of time with the fuel in it, could need a tune up - air filter, spark plug, just some Spring maintenance (like most of us :))....

Here is a link that might be useful: Mantis manual, maintenance and troubleshooting

Thanks to everyone, special thanks to Morz8 for responding.
You see, my husband is not into gardening and too busy in his business,so I am mostly responsible for "gardening part" but I was never "friends" with equipment and all technical stuff. And I just CAN NOT FIND THE MANUAL ANYWHERE. I will definately look into the link provided.

The gas is probably gumming up the carb. If you are not a good "shade tree" mechanic, you will probably have to pay to get it fixed. You won't like the bill. In the future, always add some fuel preservative to the gas tank. Ask your service guy what to buy. It's readily available, and it works. The engine runs just fine with it.

Also, most gasoline has ethanol in it nowadays. Ethanol is really tough on fuel lines, etc. Try to find some gas without ethanol in it. Marinas usually have it, or you can ask your farm/garden equipment guy where to find some. In Longview, Spirit Gas and Wilcox and Flegel sell it.