HELP PLEASE GEAR PROBLEMS

[as0102]

So, drove home last night and one of the roads had flooded with the rain. didn't look to bad that I couldn't get through but just been out to the car now and when the engine is running it won't engage any gear
when car is switched off i can put it in any gear without engaging the clutch but as soon as i start it up and press clutch down it just won't allow me to put it in any gear or reverse

has anyone else had this problem or know what it is/how to fix it