Hi Guhar,
Request you to kindly elaborate on your problem next time when you post. This will enable us to have a proper insight into the situation to offer our words.
Also, please mention "Loss of words" in which language - "Mother tongue or English."
"Loss of words" would literally mean being unable to think of something to say, i.e., unable to put your thoughts into words or simply unable to express what you know or feel.
Else, "Loss of word" term also refers to a medical weakness where people tend to forget what they know, similar to short-term memory loss (need not be afraid, this is common in everybody, only the percentage varies).
As far as the first case goes, it's an outcome of human nature due to anxiety, shock, surprise, fear, etc. So, all you need to work on is how you put your flow of thoughts in a logical manner to reason out. Initially, think of a very small passage in your mother tongue and write it down just as you thought or wanted it to be (don't do any corrections - as it would turn out to be correct most of the time). Then try to convey (don't translate) the same passage or conversation that took place in your life, etc., in the desired language and write it down. Check if the message is conveyed exactly the way you wanted. Now, go through the words carefully and see the usage of words, what words you have missed, etc., and also check the grammar. This way, you might know what you are lacking in. Then (remember), don't translate; try to find equivalent words in the desired language. Do this for a week or a little while, and meanwhile, try to learn as many words as you can. You can follow the above-said ideas from our members here.
Once you find the words to depict your thoughts, bingo, then your fear and anxiety would fade on their own.
Hey, but do respond to us what exactly you meant by "loss of words."