I have an Isotec 228 c phone system and I,m having trouble with the day ring working with incoming calls. The night ring works fine and rings through to all extensions. This problem just arose in the middle of the day. Please help!

Are you live answering the telephones or using automated attendant?If you are live answering,each telephone that you are want to ring is set by extension and per line under A station programming.You will need to access the programming through a P.C or operator console if you have one.
If you are using auto attendant to answer,again check the ringing but this is usually done by assigning the trunks to ring a group on the B screen system programming.
Then each extension for the auto attendant would be placed into the group on the A screen.Let me know which way you are using the system.

I am answering calls live and have an executone terminal to do all the programing. I just can't figure out why it suddenly quit working in the middle of the day. Also having trouble with the unfriendly interface of the system.

Do a reset. Turn off your system for about 10 min and turn it back on. It sounds like you have data failure from power bump. If that doesn't do the trick you will have to check the programming like pvj said.
Hope this helps
dtmf

Depending on the software on the CPU board,you may have access to the H screen.If you do,go to the H screen and typy L DR X where X = the line number.This will list all telephones that are programmed to ring on the line you specify.You have to be careful sometimes with setting lines to ring on a telephone and the telephone is set to call forward somewhere voicemail,another phone.

Also,don't forget to use the C screen to watch the real time activity of the system.It will show the status of lines,intercom calls and even show ringing telephones.I always thought that Executone had one of the friendliest interfaces for programming I've ever seen.