Your favourite bras can seem like they suddenly don't fit anymore for a variety of reasons - weight gain, weight loss, bloating, bands becoming less elastic over time, etc etc. If this happened recently, there are a few things that you might be able to do to make it fit again. Here are a few ideas!
Adjust the bra closure
If the band feels too tight / too loose, you can always adjust which "eye" of the bra closure you hook into. If it's still too tight, there are bra extenders you can easily buy on Amazon / any lingerie shop, to extend the band a little more
Straps
Bra straps are an oft-overlooked part of the bra that can improve fit dramatically. If the bra feels too loose, not giving you enough lift, or the bra is gaping between the bra and your body towards the top, that usually means that the bra straps are too loose. If the straps are cutting into your shoulders, then they are definitely too tight
Did neither of those things work?
I'm afraid that you might need to buy a new bra - don't put it off! Wearing the wrong size bra can lead to a whole host of health problems, and trust me, it just isn't worth it