This is rare on 6.48.6. Usually, it is due to corrupted configuration imported from a different version. Fix: Boot into NetInstall (hold reset button), do a full format, and reconfigure manually.
Add a default drop rule for input:
/ip firewall filter add chain=input connection-state=invalid action=drop
/ip firewall filter add chain=input connection-state=established,related action=accept
/ip firewall filter add chain=input protocol=icmp action=accept
/ip firewall filter add chain=input action=drop
If your router has internet access, you can download directly: Mikrotik 6.48.6 Download
/tool fetch url="https://download.mikrotik.com/routeros/6.48.6/routeros-arm-6.48.6.npk"
(Replace arm with your architecture.)
Then:
/system reboot