Can't go back using back button and can't go to task manager
So here's my problem. I've accidentally dropped my phone in the water and I've tried pressing the back and task manager button but failed.
As you know water and electronics and electricity are not a good mix.
The water causes corrosion and provides circuit paths for the electricity which were not in the phone's operating design and could damage the components.
First do not turn on your phone and then you need to remove the battery as soon as possible from the phone to minimize further damage.''
Then you need to dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 90%+ to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is only 70% and is not as effective. If you do check the label to verify the amount
Here is a link that in general, describes the process.
You may have to replace the battery as it is very hard to recover a battery from water damage.
As always with electronics, especially surface mounted PCBs be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board.
Here is a link to the Ifixit repair guide for two Oppo phones (as you do not state the model number of your phone) hopefully one of them is applicable to you.
Hopefully after you have done all this the phone might possibly work correctly again.
If this process seems too daunting, take your phone to a reputable, professional phone repair service and ask for a quote for a repair.
If you decide to do this, do it sooner than later.