Are you using latest version of magento 2 ?
Also are you using multi-site concept of magento 2?
may i know which payment method are you using?
also provide log information as well it will help to debug error
Can you please enable another payment method like bank transfer and check it.
Is your place order working, if working then it will be an issue with that payment method?
@shweta_singh1 - it seems like you have issue with either your caching system or with session lifetime.
could you please let us know are you using any caching mechanism on server ?
If yes - then kindly disable it and then check it is working or not