Tes·ti·mo·ny
noun: a formal written or spoken statement; evidence; sworn statement; attestation; affidavit; assertion, affirmation, deposition, evidence or proof provided by the existence or appearance of something.
You can have a testimony, but then not keep that testimony just like you can have a gift but then lose it.
God commands us to "Diligently KEEP the commandments of the Lord your God, HIS TESTIMONIES, and His statutes, which He has commanded you... that it may be well with you, and that you may go in and possess the good land of which the Lord swore to your fathers." Deuteronomy 6 :17, 18
They were to KEEP what He gave them so that it would go well with them. This didn't mean to keep it to themselves. It meant to recount it to each other and especially to their children. They were to build a monument on the other side of where they crossed the Red Sea and escaped annihilism from the Egyptian Army. Why? So that when a child said, "Hey Grandpa, why is there this great big pile of stones here?" He would then say, "Well, this is the exact spot where millions of us crossed on dry ground and God redeemed us from slavery."
That's not only the power of the testimony, but that is the keeping of the testimony. They were to repeat the evidence of God's mighty wonders over and over to every generation.
But then throughout Psalm 78 we read of when God's people actually forgot His works and His wonders which He performed for them in order to set them free and to provide for them. They didn't keep the amazing testimonies, the evidence of His greatness. Then calamity struck them. They stopped recounting God's wonders and mighty deeds to each other and to their children. They stopped being a witness to the evidence they experienced. It was devastating!
Jesus said that we would receive power when the Holy Spirit comes upon us in order to be WITNESSES in our own city and to the world.
Acts 1:8. But you can't witness what you haven't encountered and you can't encounter without the presence of Holy Spirit.
If you have a testimony of God's mighty wonders, then you are to retell it over and over until you have another testimony. Then retell that until you have another. If you don't have a testimony, ask God for one or tell someone else's testimony until you have your own. Never stop witnessing God's power. It's a command that all may go well with you.