There appears to be an issue with your client's account. Communicate with your client to resolve any issues they have on their end and do not continue working until the contract is active again.
My client and I agreed to end the contract, and I should get the half of the agreed amount - 75$, which is funded.
But the client is not answering to my messages...
What should I do next?
Click on "submit work and request payment" on the contract, and edit the requested amount.
This is assuming that the contract / milestone is funded to at least as much as you agreed to be paid.
If the client doesn't respond to your release request, Milestone funds will be released to you automatically 14 days after the most recent Milestone submission.