1. if it is soleniod, is it posible that it works again for few times? YES, this is possible
2. i have a spare similar soleniod but it is written 12v, can i try this one? NO, your bow thruster is 24 volts and I assume that all of the controls are 12 volts
3. i thought maybe it could be the switch. there are four cabels connected. two lefe and two right. These cables are switch cables. on each side of the switch you have 2 cables connected to the switch...activating the switch causes these two cables to connect. So one set of two cables for PORT thrust and one set of two cables for STARBOARD thrust. On each side of the switch, one will be hot (+24V) with the switch in neutral, then with the switch moved either left or right, the other wire on the same side will become connected to the first wire and be hot (+24V).
If I was checking this, I would find a good 24 volt ground to connect the negative cable to the voltmeter, then checking one side at a time find the hot terminal on each side of the switch. Then move the switch to see if the switch turns on the other terminal on each side. You can also turn the power off to the switch and check continuity (connection), but that will not tell you which switch terminal is the input and which one is the output. I think that the switch is probably OK because the switch will not wear out with use like a solenoid will.
So simply stated your bow thruster has two switches that have to operate for PORT and two for STARBOARD.
The first switch for PORT will be at the joy stick in the cockpit. That switch sends 24 volts to the electromagnet inside the PORT solenoid. When the electromagnet is engaged, another switch connects inside the solenoid. This last connection will be a high amp connection with more than 100 amps flowing through the connection...that is enough amperage to weld some metals. So every time you move the joy stick to PORT inside that solenoid there is some burning at the contact points. As the burning gets worse, the contact gets worse and eventually it connects some of the time, then not all. The same thing happens to the STARBOARD solenoid when it is engaged.
Maybe you can understand why I assume that it is the solenoid?
I will be leaving for the airport in a few minutes and be off line for about 10 days. I suggest that you contact Riza at Emek Marine.