Have you configured your router to assign your computer a specific IP address?
If not, then your IP address most likely changed because a DHCP lease expired. One of the uses of DHCP – the Dynamic Host Configuration Protocol – is automatically assigning IP addresses to devices on the network. Addresses are "leased" for a length of time, and your computer will request a new lease once the one it currently holds expires.
If you're using ISP-provided equipment, it will almost certainly have come with DHCP enabled, as it enables zero-effort set-up, and unless you specifically configure your router to assign your computer a specific IP address, you will get a random address from the pool the router has been configured to use. In your case, this pool is probably 192.168.1.0/24.
To set your router to assign you a specific IP address, look for something labelled DHCP Settings or DHCP Reservation in the router's administration interface. You will most likely then be prompted to enter a MAC address and the desired IP address. Your router may display the MAC addresses of currently-connected devices, or you may have to enter it manually. To find your MAC address, enter ipconfig /all
into the command prompt and look for "Ethernet adapter" sections:
![ipconfig /aa](https://cdn.statically.io/img/i.sstatic.net/NMWSx.png)
The value marked in red in this image is the one you want (although I've censored three octets of mine).