I've been using Essentials on my server and I had to switch computers because my other one had lost most of its memory, but on this computer for some reason essentials doesn't seem to work. I put a quick server up so me and my cousin could play together a few days ago and it worked fine. This is the message my terminal gives me every time I try to start my server:

18:34:59 [SEVERE] Could not load 'plugins/Essentials.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: Bad version number in .class file
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:151)
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:305)
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:230)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:213)
at org.bukkit.craftbukkit.CraftServer.<init>(CraftServer.java:189)
at net.minecraft.server.ServerConfigurationManager.<init>(ServerConfigurationManager.java:53)
at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:166)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:432)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
Caused by: java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:676)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:41)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
at java.lang.ClassLoader.loadClass(ClassLoader.java:317)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:375)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:242)
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:140)
... 8 more