They prevented me - They anticipated me, or went before me. See the note at Psalms 18:5. The idea here is that his enemies came before him, or intercepted his way. They were in his path, ready to destroy him.
In the day of my calamity - In the day to which I now look back as the time of my special trial.
But the Lord was my stay - My support, or prop. That is, the Lord upheld me, and kept me from falling.