Safari will not open unless computer is booted in safe mode.

Try this: Instead of starting the entire computer in Safe Mode, start Safari in safe mode by holding SHIFT while launching Safari. If Safari works that way, you have incompatible extensions installed. While in Safari Safe Mode, go to Safari preferences > Extensions and turn off all extensions. Then quit Safari and see if it opens normally.

If it does, you can turn extensions back on one at a time and test. When the problem returns, you have found the bad extension.

Good Safari performance also means NOT running anti-virus or so-called "cleaning/tune-up/optimizing" software.

There is also an application called Trusteer Endpoint Protection or Trusteer Rapport that banks recommend people install on their computers for secure banking, not knowing it mangles Macs. Did your bank ask you to install a security package?

leahreid1 wrote:

When I try to open Safari back up again, a very long error message popped up.

What kind of message is this? Is it a crash report? If so, you probably have the Genieo adware installed. It has been causing a lot of Safari crashes lately. See:

Genieo adware causing Safari crashes

If instead it's a long message telling you something about being infected with malware or your computer being locked by law enforcement, that's a scam. See:

Tech support scam pop-ups

(Fair disclosure: I may receive compensation from links to my sites, TheSafeMac.com and AdwareMedic.com, in the form of buttons allowing for donations. Donations are not required to use my site or software.)

There is no need to download anything to solve this problem.

If Safari crashes on launch and you don't have another web browser, you should be able to launch Safari by starting up in safe mode.

You may have installed the "Genieo" or "InstallMac" ad-injection malware. Follow the instructions on this Apple Support page to remove it.

Back up all data before making any changes.

Besides the files listed in the linked support article, you may also need to remove this file in the same way:

~/Library/LaunchAgents/com.genieo.completer.ltvbit.plist

If there are other items with a name that includes "Genieo" or "genieo" alongside any of those you find, remove them as well.

One of the steps in the article is to remove malicious Safari extensions. Do the equivalent in the Chrome and Firefox browsers, if you use either of those.

After removing the malware, remember to reset your home page in all the web browsers affected, if it was changed.

If you don't find any of the files or extensions listed, or if removing them doesn't stop the ad injection, then you may have one of the other kinds of adware covered by the support article. Follow the rest of the instructions in the article.

Make sure you don't repeat the mistake that led you to install the malware. Chances are you got it from an Internet cesspit such as "Softonic" or "CNET Download." Never visit either of those sites again. You might also have downloaded it from an ad in a page on some other site. The ad would probably have included a large green button labeled "Download" or "Download Now" in white letters. The button is designed to confuse people who intend to download something else on the same page. If you ever download a file that isn't obviously what you expected, delete it immediately.

In the Security & Privacy pane of System Preferences, select the General tab. The radio button marked Anywhere should not be selected. If it is, click the lock icon to unlock the settings, then select one of the other buttons. After that, don't ignore a warning that you are about to run or install an application from an unknown developer.

Still in System Preferences, open the App Store or Software Update pane and check the box marked

Install system data files and security updates (OS X 10.10 or later)

or

Download updates automatically (OS X 10.9 or earlier)

if it's not already checked.