Short answers, pulled from the story.
Germany demanded that Lithuania transfer the Klaipėda Region, also known as the Memel Territory, to the German Reich. Foreign minister Joachim von Ribbentrop delivered the demand orally to Lithuanian foreign minister Juozas Urbšys on the 20th of March 1939, threatening a Wehrmacht invasion and the bombing of Kaunas if Lithuania refused.
Lithuania accepted the ultimatum on the 23rd of March 1939 because it had no material international support. The United Kingdom and France followed a policy of appeasement, Italy and Japan openly backed Germany, and the Soviet Union declined to act. Lithuanian diplomacy described the concession as a "necessary evil" to preserve the country's independence.
Klaipėda handled between 70% and 80% of Lithuania's foreign trade and, though it made up only about 5% of the country's territory, it contained roughly a third of its industrial output. By the end of 1939, Germany accounted for 75% of Lithuanian exports and 86% of Lithuanian imports, pulling the country into the German economic sphere.
No. The ultimatum was delivered orally by Ribbentrop during a roughly 40-minute meeting with Urbšys on the 20th of March 1939 and was never set down in writing. It also carried no formal deadline, which led some historians to describe it as a "set of demands" rather than a true ultimatum, though the threat of military force was explicit.
The 1924 Klaipėda Convention, signed by four powers, was designed to guarantee the status quo of the Klaipėda Region. When Germany issued its ultimatum, Lithuania secretly notified the other signatories, but none offered material assistance; the convention provided no practical protection.
Acceptance of the ultimatum deepened dissatisfaction with President Antanas Smetona's authoritarian rule. The cabinet of Vladas Mironas was replaced by one led by General Jonas Černius, and for the first time since the 1926 coup opposition politicians entered government, including Leonas Bistras as Minister of Education and Jurgis Krikščiūnas as Minister of Agriculture.