2 Chronicles 7:14 has been used for decades as a prayer model for the repentance and spiritual healing of a nation with zero effect. Ever wonder why?
"and My people who are called by My name humble themselves and pray and seek My face and turn from their wicked ways, then I will hear from heaven, will forgive their sin and will heal their land."
First, as always, we must look at the context, which in this case is easy. In verse 13, God lays out three separate hypothetical situations given in the form of "if":
"IF I shut up the heavens so that there is no rain, or IF I command the locust to devour the land, or IF I send pestilence among My people"
So, first, we are dealing with hypotheticals. Second, notice that each of these hypotheticals deal strictly with the physical, not spiritual matters.
The Partial Answer: This scripture is always used in dealing with spiritual issues, not physical - AND the entirety of these verses deal with hypotheticals and not actual events...and how can we expect to get real answers to prayer about a non-real issue? But even this doesn't get to the actual heart of the matter...
The Full Answer: You cannot repent for any other sin except your own. If that were true, you could repent for your brother's sins and he'd be forgiven and saved, but you can't. Matthew 6:15 deals with this - the key words being "you" and "your".
Both Daniel and Nehemiah 'confessed' the sins of the nation - Neh 1:5-7 and Dan 9:1-19 - but at no time did they attempt to repent for those sins because they understood that it can't be done.
The Bottom Line: We can only repent for what we ourselves have done - what we ourselves are personally responsible for. We can pray that the Holy Spirit would convict others to do the same, but we cannot take responsibility for the sins of others, either past or present, and reconcile them to the Lord.