"brother mfc 7840w printer goes offline and i cannot print need turn printer on and off.

The solution, and it took me a long time to find the simple solution, is to right click on your printer, click properties, click the ports tab and click on your Brother printer and click on configure port (or you can delete the existing one and then add another one). What you want to do is update the IP Address. The problem is that the printer does not have a static IP address, it is dynamic. If you want to confirm that, go to your printer and press the reports button and print out the network configuration report. Your IP address will show on that. It's really the last digit in the IP address that changes. This will absolutely solve your problem. Good Luck!

Dmarciante is absolutely right, sometimes the IP address changes. However, here is another reason you may see the printer as off line, you move around your laptop to different network (for example from home to office network). To see if this is your issue, right click on the printer (from the Printers and Faxes window for example) and see if you have a menu option that says "Use Printer Online". If so just click on that option and you are done. If this fails try the IP address resolution. See the image below...
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