No longer can access web server from outside local network

Question

I'm running XAMPP locally on a Windows machine. For months I've had no trouble with my setup, but all of the sudden yesterday, anyone outside of my local network became unable to access the site. Anyone trying to access a page from outside the network now gets a 504 Gateway Timeout error. I'm not a pro with Apache and am totally confused as to what is wrong and why it didn't happen until now. Here are the things I know (or am pretty sure) aren't causing the problem:

• Apache just not working (I can connect locally)
• A problem with DNS resolution (The site can't be accessed by IP either)
• A problem with firewalls (neither Windows or my router's are blocking connections)
• A problem with port forwarding (These settings worked before and I haven't changed them, I have to assume they should still be right)
• Apache needing restarted.
• Nothing helpful in the Apache error.log.

This local server is only temporary, but it may be a month or so before I can get set up on a more permanent one. I would really appreciate if someone could help me peg down what is causing this issue.

Edit:

The most recent change I've made to the server is changing the permissions on the /tmp/ folder. I was having trouble with users not having the ability to upload files to the server, so I removed the read-only flag on that folder. The issues arose about 18 hours after this.

Answer 1

After getting some great help in the comments, I learned that my router was no longer forwarding the connection to the correct machine on my local network. I'm still not sure how this happened without my changing the port forwarding settings, but next time my site is inexplicably down I'll be double checking that the correct port is open first.