$1400 of flight credits gone

Not even 2 months ago I had 1400 in flight credits that I had at least half a year left till they expired, Checked today and nothing. I know it wasn't just a glitch in the app or something either as I could use them in the website too book flights etc. I talked too an agent over the chat on the website but they were no help and told me they had no record of me ever having them. What should I do as I was planning on visiting my dying grandmother using them but now I can't. United is a disgrace and needs too get theyre s*** together. Any advice in how I could resolve this or anyone else whose had too deal with this before would be much appreciated.