Such machine can be rescued by cleaning and drying, probably with some losses concerning the harddisk drive.
Never try to start a wet computer. Make sure the computer is disconnected from any (line) voltage supply and remove any battery. The battery will be discharged and has a bad tendency to leak corrosive agent.
Dismantle the computer to remove traces of mud trapped between parts, using a shower.
Soak and rinse everything in deionised water (as available for steam ironing).
Dry everything quickly and completely; best in a container together with an appropriate amount of dried silica gel, if not available with a fan. Do avoid excessive use of heat, as you may damage plastic parts. Afterwards check for visible damages, reassemble the poor thing and try if it will work again.
The hard disk drive will need separate treatment if it is affected from the water and the data on the platter is of interest. In case the data is very valuable, immediately call for professional data recovery. In case you do not mind, dump the drive and get another one. In case you will try to rescue data on you own, try this: open the disc drive case (preferably in a clean, dust free environment). Do not touch or dismantle any of the movable parts inside. Soak and rinse it completely with deionised water. Dry it quickly and completely in a container together with dryed silica gel, but no fan, as it would contaminate the drive with loads of dust. Reassemble the dried drive and connect it to a computer prepared to immediately make a backup of the stored data (in case you are lucky and the drive mounts).
Good luck
P.S.: Drying the machine uses time, let it be in the closed drying container for a week and make sure the water absorbing agent itself is working, i.e. enough of that stuff and dry enough to work. I successfully used some pounds of the silica gel bags used in the computer equipment boxes for shipping and storing. You can get vast amounts of this stuff in computer shops which sell customized PCs.
