This happened to me a few years back. Made Silver, enjoyed Silver benefits the following year while redeeming a large bank of miles, missed making Silver by a little bit due to inattention when trip planning, and had to start all over (pre-Bronze) the next year. I travel on my own dime so I try to make the most of my miles and $$.

Taught me to pay closer attention to my TP balance and tier expiration date. Also taught me I REALLY like Silver benefits.