Need Serious Help

I bought a boat today and when i got it in the water today it ran finewith no problems at all and when i got back to the boat ramp i put it in reverse and now i cant get it in neutral or anything it wont come out of reverse at all and like i said it ran fine not even 10 minutes before hand i went forwards stopped went backwards and went back to forwards but when i thought all was well thats when it got stuck in reverse its a 40 horse yamaha im not sure what year model is there any mnor thi g it could be i could check. Like i kinda need a check list of what i can check. Iv never worked on boat motors before but i didnt know if it could be a cable out of adjustment o what and it is console steering

no idea on that motor but they're typically pretty simple, either the cable is seizing up, or the throttle or the shift lever the cable attaches to at the engine. On my old 150 i removed the cable, actuate the throttle, if it shifts the cable is fine. Next see if you can budge the lever at the engine (mine has a zerk fitting) doubt most ever grease them till there is a problem. If you can't move it, grease the heck out of it. Can't really tell you more that's all i've ever ran into on my boats.

Take a look at the shift rod down by the lower unit. Is it rotted and weak?