As Mark said, if it worked before you made a change, and didn't work afterward, chances are the change you made has caused it to not work. Because we don't yet know you, your mower, or the changes you made, we need to work through this in an orderly manner.
Please post pictures of your mower, in particular of the left side of the engine where the controls and adjustments are all located. Since you have just changed the air cleaner's filter element, please remove the top of the air cleaner housing and include pictures of the inside of the housing, with the element installed and with it removed. Try to start the engine with the top off and the filter removed. If it starts, stop it immediately - never run the engine without the filter properly assembled.
