On the low power end ( under 13W) you find iron core choke ballasts that are multi rated 7,9,10,11,13W which will work fine with all lamps in that power range, and a nominal 20w ballast works with 18W tubes as well, and with 16W low power tubes as well, though there the efficiency comes at a cost of needing an electronic starter to limit the restrikes at EOL and also provide a longer preheat time.
however in the US with 120VAC your choke ballast is not so generously rated, and most likely has failed open circuit if choke type, or has a broken connection on the terminals ( pretty common). If electronic then rather than buy a new one simply look for an "angel eye' circle of LEDs online, and a small 12V power brick to power it. Sold as car light rinds in assorted sizes, and a lot lighter than the circline tube and ballast, though you probably want to leave the existing choke ballast in the base to keep the light base heavy.