This article assumes that you had already setup your card reader before and that it was working properly up until now.
Please make sure your tablet and the card reader is connected to the same internet/wifi network. Your tablet might have switched to another network without your approval.�
Android remembers the networks that you have authorised and so if two or more authorised networks are in range then the tablet will switch between them depending on signal strength.
If this is the case then please make sure to make your tablet forget the network that you don't want it to use. There is unfortunately no way to lock android on to a single network, so the only way is to not have any other authorised networks. This includes if you have in case of emergency connected the tablet to a personal hotspot.
Please make sure your card reader is connected to a network. You might have changed your router, internet provider or even the password on the router. This happens more often than you think. If so, then please call the card reader company. Their phone number is on the device.
Please make sure your card reader is on a static (fixed) ip address and that no other device can obtain the same ip address. If you suspect that the card reader has changed its ip address then you can look into the connect to card reader process.
If you don't have an enterprise grade router or the router has aged a little and might have been running for months at a time then you might want to try to restart your router. Sometimes routers malfunction in a way that they fail to assign static ip's.