Toddler cries for no reason!

What do you do when a toddler ALWAYS cry for no reason?! I’m actually losing it here and it’s driving me crazy. My toddler is 18 months and she just cries and cries continuously for no reason whatsoever. Parenting is just so hard especially when you are doing it alone 😭 she done it again this evening and nothing I did was making her stop crying... I danced and danced, sang all the songs I could think off, rocked her until my arms almost fell off but still she continued crying. I just lost it and started crying. She isn’t teething and no marks