The thing is, Zimbabwe is a wealthy country, but two things screwed over Zimbabwe.
The first thing is that all real business in Zimbabwe buys and sells in US dollars, Euros, or other African currencies. That means that despite Zimbabwe's multi-billion dollar economy, whatever Zimbabwean money people end up with they will immediately try to get rid of in exchange for American, European or South African currency.
The second is that the government of Zimbabwe is enormous, it spends several billion dollars every year maintaining it's enormous and bloated bureaucracy through levying taxes on income and business.
Zimbabwe's economy was in decline during the 1990s due to various factors, including lessening demand for their exports. Zimbabwe's land reform, giving prime farmland to poor farmers from rich white ones, was not the death-blow people make it out to be, but Mugabe had also been keeping his government afloat on IMF debt for years and had racked up over $5 billion.
In response to Mugabe's land reform, the USA basically cut them off from all IMF funding with the
"Zimbabwe Democracy and Economic Recovery Act of 2001", and the IMF called in their loans.
Mugabe's answer to retaining the size of his government, not giving into the demands of the international community and paying off the loans all at the same time was to print money. Printing money is essentially taking value away from existing money and then spending it, it's a form of taxation. The thing is, a large percentage of the population employed by the enormous government which gets paid in Zimbabwean dollars gets a piece of that income removed every time he prints another piece of Zimbabwean currency. Losing your national currency's stability is a very risky thing, and he should have pursued other avenues before going down that road.
Mugabe needs to drastically slash the expenses and bureaucracy and general bloated size of his government, and get the IMF off his back. Until he does his country will continue to spiral into ruin.