Hi... My car was standing in my garage for about 3 week's and in the cooling system was only demineralized water. The weather temperature outside the garage was about - 5 degC and in the garage there was about -1 degC. After this 3 weeks I wanted to start my engine and I check'ed how much water was there in the cooling system. And I realized that there is no water in the system;/ I looked under my car and saw this think:
I think it is this part that is arowed:
Why is this plug jumped out the block? what function has it? and can I plug it again in the engine block?

I think it is this part that is arowed:

Why is this plug jumped out the block? what function has it? and can I plug it again in the engine block?