If your sand filter is clogged up, your backwashing cycles will get shorter and shorter and shorter. Check to see if the sand filter looks dirty or greasy. If it looks like sandy lard, water is definitely going to stop flowing through it. Backwashing the sand filter isn’t really based on time but on pressure. Backwash the filter once the pressure gets to be 8-10 psi over the starting level. If any of this is over your head, contacting a pool repair specialist who provides this type of service is probably your best bet.